SERIAL killings like the ones generating headlines out of Milwaukee the past two weeks are a relatively rare form of murder, say criminologists.But serial killings are on the rise, and all-too-common social and cultural problems are believed to be a factor in what drives human beings to this violent extreme, they say. An understanding of the underlying meaning of the killings to the offenders themselves is key to solving murders as well as preventing them, say criminologists. Trying to make sense of the apparently senseless, to understand the killers, they say, will help police shorten the killers' careers if not prevent them altogether. "We can't relegate the whole thing to evil; to inhumanity. We have to think about the themes that excite [the serial killer]," explains Candice Skrapec, a City University of New York criminal psychologist whose research involves extensive interviews with people incarcerated for serial killings. The consensus among experts in the field is that it is not coincidental that a sudden upturn in the late 1970s of serial killings in the United States - as well as other violent crimes - paralleled increases in dysfunctional families, reported child abuse, media images of violence, and sexual and psychological permissiveness. The social milieu plays no small part in the escalation of serial killings, experts suggest.